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Microplastic contamination of the oceans is one of the world’s most pressing environmental concerns. The terrestrial component of the global microplastic budget is not well understood because sources, stores and fluxes are poorly quantified. We report catchment-wide patterns of microplastic contamination, classified by type, size and density, in channel bed sediments at 40 sites across urban, suburban and rural river catchments in northwest England. Microplastic contamination was pervasive on all river channel beds. We found multiple urban contamination hotspots with a maximum microplastic concentration of approximately 517,000 particles m−2. After a period of severe flooding in winter 2015/16, all sites were resampled. Microplastic concentrations had fallen at 28 sites and 18 saw a decrease of one order of magnitude. The flooding exported approximately 70% of the microplastic load stored on these river beds (equivalent to 0.85 ± 0.27 tonnes or 43 ± 14 billion particles) and eradicated microbead contamination at 7 sites. We conclude that microplastic contamination is efficiently flushed from river catchments during flooding.

The placement of wood in rivers is a common restoration method used to locally affect hydraulic and morphologic conditions to create habitat. Laboratory experiments demonstrated that wood placements can also promote surface declogging, that is, removal of fine sediment from a gravel bed, thereby restoring spawning grounds for fish. Logs of different size and submergence level were placed on a gravel bed clogged with fines. Surface declogging was observed in regions of elevated turbulence in the log wake and elevated velocity adjacent to the log. A criteria for declogging was identified based on a modified non‐dimensional Shields parameter combining mean and turbulent velocity at the bed. The footprint of declogged bed scaled with log dimensions. Emergent logs produced a larger declogging footprint compared to submerged logs of the same length, due to their stronger influence on the flow field. Logs were also shown to prevent clogging over similar areas. Plain Language Summary Placing wood in rivers is a common way of improving river health and creating regions for fish and small aquatic animals to live. This method involves the placement of individual wood pieces (logs) at the side or in the center of a river. In this study, we explored how the positioning of logs could be used to remove fine sediments from gravel beds by changing the way water flows. This is important because excessive deposition of fine sediments in the bed, defined as riverbed clogging, can make the channel bed too compact. This makes it harder for fish to find good spots to lay their eggs. Using laboratory experiments, we observed that the logs generated regions with strong water turbulence (swirling) and faster flowing water in their surroundings, both of which removed fines from the gravel bed surface. We identified specific conditions necessary for effective removal of fines, which were related to both the turbulence and water velocity. The size of the cleaned area on the gravel bed was proportional to the size of the log. Key Points Deliberate wood placement can promote declogging adjacent to and downstream of logs Declogging was correlated with elevated turbulence downstream of logs and elevated velocity adjacent to logs Emergent logs declogged a greater surface area than submerged logs

Geogenic groundwater arsenic (As) contamination is pervasive in many aquifers in south and southeast Asia. It is feared that recent increases in groundwater abstractions could induce the migration of high-As groundwaters into previously As-safe aquifers. Here we study an As-contaminated aquifer in Van Phuc, Vietnam, located ~10 km southeast of Hanoi on the banks of the Red River, which is affected by large-scale groundwater abstraction. We used numerical model simulations to integrate the groundwater flow and biogeochemical reaction processes at the aquifer scale, constrained by detailed hydraulic, environmental tracer, hydrochemical and mineralogical data. Our simulations provide a mechanistic reconstruction of the anthropogenically induced spatiotemporal variations in groundwater flow and biogeochemical dynamics and determine the evolution of the migration rate and mass balance of As over several decades. We found that the riverbed–aquifer interface constitutes a biogeochemical reaction hotspot that acts as the main source of elevated As concentrations. We show that a sustained As release relies on regular replenishment of river muds rich in labile organic matter and reactive iron oxides and that pumping-induced groundwater flow may facilitate As migration over distances of several kilometres into adjacent aquifers.The interface between riverbed and aquifer is a biogeochemical reaction hotspot for arsenic release from river sediments, according to numerical simulations of groundwater flow and biogeochemical reaction processes.

Forecasting flood and drought events requires accurate modeling tools. Hydraulic river models are based on estimates of riverbed geometry which are traditionally collected in situ. The novel Ice, Cloud and Land Elevation Satellite 2 [ICESat‐2] lidar altimetry mission with 6 simultaneous high‐resolution laser beams provides the opportunity to define river cross‐section geometries as well as observe water surface elevation [WSE] and water surface slope spatially resolved along the river chainage. This paper describes a method to utilize terrain altimetry and water surface slope estimates to define complete river geometries from ICESat‐2 data products, using the diffusive wave approximation to calculate depth in the submerged section not penetrated by the lidar. Exemplifying the method, cross‐sections are defined for a stretch of the Mekong River. Hydrodynamic model results of the stretch are compared with ICESat‐2 WSE estimates and in situ gauging station time series. Insights in river characteristics from satellite imagery and the ICESat‐2 slope estimates allow for fine‐tuning of the cross‐sections using spatially varying Manning numbers. The final model achieves a root mean square error against the ICESat‐2 WSE of 0.676 m and average Kling‐Gupta Efficiency against gauging station time series of 0.880. The method is limited by the diffusive wave approximation resulting in inaccurate cross‐section estimates in sections with supercritical flow or significant acceleration. Errors can be identified from ICESat‐2 WSE estimates and reduced with additional cross‐sections. Combined with hydrological models, the method will allow for cross‐section definition without in situ data. Plain Language Summary The depth and width of the river channel are important factors when seeking to predict floods. To predict water level in a river, computer models for flood forecasting must be informed with river channel geometry at cross‐sections along the river. Such cross‐sections are traditionally measured in the field which is expensive and time consuming. This paper describes a method to estimate river cross‐sectional shape from land and water height measurements from the satellite ICESat‐2. When the satellite path crosses the river, the precise laser instrument onboard outlines the surface to indicate the river channel shape, but it does not penetrate the water. We found that it was possible to estimate the shape of the submerged part of the cross‐section using the water surface slope obtained from the satellite's unique instrument. Evaluating the method on a stretch of the Mekong River, we were able to model water level along the river with accuracy sufficient for flood forecasting. In some sections, where the flow speed changes quickly, a hydraulic model with the defined cross‐sections does not reproduce observed water levels. Using satellite crossings from days with steadier flow reduces the error. The method allows for building hydraulic river models without cross‐section surveys. Key Points Land elevation and water surface slope estimates from the ICESat‐2 lidar altimetry mission are combined to define river cross‐sections RMSE of 0.676 m and KGE of 0.88 is achieved for a hydrodynamic model of a stretch of the Mekong River using the derived cross‐sections Sections with significant flow acceleration lead to inaccurate cross‐sections, but errors are reduced with additional ICESat‐2 crossings

River restoration projects often involve vegetation planting to retain sediment and stabilize riverbanks. Laboratory experiments have explored the impact of rigid emergent vegetation canopies on bed morphology. Inside canopies, bed erosion is attributed to vegetation‐induced turbulent kinetic energy (TKE). Based on the in‐canopy local TKE and the criteria for sediment movement, a method is established and validated for predicting the length of the bed erosion region. In the bare channel, bed erosion is related to the ratio of canopy length to flow adjustment distance, L/LI, and exhibits two trends. At L/LI < 1, the maximum depth, ds(bare), and length, Ls(bare), of the bed erosion region increase with increasing canopy length. At L/LI ≥ 1, ds(bare) and Ls(bare) are not influenced by the canopy length and remain constant. In vegetated regions with the same length and plant density, discontinuous canopies (streamwise interval s ≥ canopy width D) yield weaker bed erosion than continuous canopies. The mutual influence between two canopies must be considered if the canopy interval satisfies s < 3D. These results provide insights for designing vegetation canopies for river restoration projects. Key Points The impact of canopy length on bed morphology inside and outside the canopy is clarified A method for predicting the length of the bed erosion region inside canopies is established Discontinuous canopies produce weaker bed erosion than continuous canopies for the same vegetated region length and plant density

The hyporheic zone is the interface between surface water (SW) and groundwater (GW) in shallow aquatic sediments where reactions can attenuate contaminants. Dunes that drive hyporheic exchange in sand‐bedded rivers constantly move (translate), causing “turnover exchange,” yet few numerical studies of hyporheic processes account for this motion. Furthermore, microbial communities that mediate contaminant reactions are constantly adjusting to their environments, including to effects of migrating sediment, but prior studies have not examined the combined effects of migrating dunes and microbial growth/death. We coupled SW hydrodynamics (OpenFOAM), GW hydraulics (MODFLOW), and GW reactive transport and microbial growth/death (SEAM3D) models to simulate the effects of dune translation and dynamics of aerobic microbial colonies on subsurface transport and consumption of dissolved oxygen and dissolved organic carbon (DOC). Dune translation was implemented by modifying SEAM3D to incorporate a moving frame of reference. As dune translation speed (celerity) increased with increasing SW velocity, turnover exchange, influx of DOC from SW, aerobic microbial growth, and DOC consumption all increased, given transport‐limited conditions. Our no‐growth models predicted only half the DOC consumption as the growth/death models despite having over six times the biomass. Explicitly simulating microbial growth/death allows simulated microbial populations to more efficiently process DOC by adjusting their spatial distribution to substrate patterns. This effect multiplies as turnover exchange increases with dune translation, highlighting the reinforcing effects of dune movement and microbial dynamics. Our results underline the importance of including both translation and growth/death dynamics when simulating hyporheic transport and reaction induced by riverbed dunes.

In rivers worldwide, multiple scales of dunes coexist. It is unknown how the larger, primary dunes interact with secondary bedforms that are superimposed. We test the hypothesis that streamwise variability in the sediment flux inferred from the downstream migration of secondary bedforms explains migration of the host dune, based on bathymetric data from a lowland, sand‐bedded river. Results indicate that transport estimated from secondary bedform migration increases along the host dune stoss, eroding the stoss slope. When the superimposed bedforms disintegrate at the primary lee slopes, results indicate that all sediment transport associated to secondary bedform migration is arrested in the lee of the host dune, explaining migration of the host dune. When secondary dunes persist however, only part of the sediments transport linked to secondary dunes contributes to the migration of the host dune. This study gives novel insight into the fundamental mechanisms controlling the kinematics of compound dunes. Plain Language Summary Dunes are undulating features that can develop on a sandy river bed. They migrate downstream as a result of sediments moving from the stoss, the upstream facing slope of the dune, to the lee, the downstream slope of the dune. Sometimes, multiple scales of dunes coexist, where trains of small dunes travel over larger dunes. In this study we investigate how two dune scales interact and how they contribute to the downstream transport of bed sediments. This is done based on a series of field campaigns in the River Waal. The results indicate that migrating secondary dunes contribute to the displacement of the host dune, the dune over which they migrating. In some cases, secondary dunes travel over the host dune stoss and disintegrate at the host dune lee, depositing sediment there. In other cases, secondary dunes travel over the full length of the host dune toward the next, downstream dune. In this case, part of the sediments transport linked to the secondary dunes contributes to the downstream displacement of the host dune, and part of the sediments are transported to the next primary dune. Key Points Secondary bedforms are omnipresent and are key to understanding primary dune behavior Sediment transport rates linked to migration of superimposed river bedforms increase over the host dune stoss and decrease over the lee side Secondary bedforms control migration of the host dune, both when they persist over the host dune and when they disintegrate at the lee side

Grain size analysis is the key to understand the sediment dynamics of river systems. We propose GRAINet, a data-driven approach to analyze grain size distributions of entire gravel bars based on georeferenced UAV images. A convolutional neural network is trained to regress grain size distributions as well as the characteristic mean diameter from raw images. GRAINet allows for the holistic analysis of entire gravel bars, resulting in (i) high-resolution estimates and maps of the spatial grain size distribution at large scale and (ii) robust grading curves for entire gravel bars. To collect an extensive training dataset of 1491 samples, we introduce digital line sampling as a new annotation strategy. Our evaluation on 25 gravel bars along six different rivers in Switzerland yields high accuracy: the resulting maps of mean diameters have a mean absolute error (MAE) of 1.1 cm, with no bias. Robust grading curves for entire gravel bars can be extracted if representative training data are available. At the gravel bar level the MAE of the predicted mean diameter is even reduced to 0.3 cm, for bars with mean diameters ranging from 1.3 to 29.3 cm. Extensive experiments were carried out to study the quality of the digital line samples, the generalization capability of GRAINet to new locations, the model performance with respect to human labeling noise, the limitations of the current model, and the potential of GRAINet to analyze images with low resolutions.

The hydrological and biogeochemical properties of the hyporheic zone in stream and riverine ecosystems have been extensively studied over the past two decades. Although it is widely acknowledged that sediment heterogeneity can influence biogeochemical reactions, little effort has been made to understand the role of heterogeneity on the spatiotemporal variability of riverbed redox conditions under changing flow dynamics at different timescales. Here we integrate a mechanistic model and field data to demonstrate that heterogeneity in permeability plays a vital role in modulating sediment redox conditions at both seasonal (annual) and event (daily‐to‐weekly) timescales, whereas heterogeneity in particulate organic carbon (POC) content only has a comparable influence on redox conditions at the seasonal timescale. These findings underscore the importance of accurately characterizing sediment heterogeneity, in terms of permeability and POC content, in quantifying biogeochemical dynamics in the riverbed and hyporheic zones of riverine ecosystems. Plain Language Summary The redox condition of riverbed sediments is subject to the combined influence of hydrologic exchange flow and biogeochemical processes and is important for regulating the functioning of riverine ecosystems. Current understanding of the spatiotemporal pattern of sediment redox conditions especially with heterogeneity in consideration is limited, partially due to the lack of measurements and quantitative models. In this study, we integrate a mechanistic model and field data to reveal the role of sediment heterogeneity in controlling the redox condition under dynamic flow conditions. We demonstrate that heterogeneity in permeability modulates sediment redox condition at both seasonal and event timescales, and heterogeneity in particulate organic carbon is most prominent over multi‐month time intervals that reflect the balance between particulate organic carbon (POC) metabolism and time‐integrated oxygen influx. These findings highlight the importance of accurate characterization of sediment heterogeneity in both permeability and POC for predicting the dynamic redox shifts in riverbed sediments. Key Points A reactive transport model was developed to quantify the impact of heterogeneity in permeability and particulate organic carbon (POC) concentration on sediment redox conditions Heterogeneity in permeability controls sediment redox conditions at both seasonal (annual) and event (daily‐to‐weekly) timescales The effects of heterogeneity in POC occur over the monthly timescale, reflecting a balance between POC metabolism and the influx of oxygen

Landslides are natural hazards that represent a huge economic burden and cause the loss of human life around the world. In countries such as Colombia, the mass movement events that cause the highest number of deaths and economic losses are often related to river or stream flooding caused by landslides in basins. Therefore, it is necessary to develop tools that estimate and assess landslide risk in such areas. This study presents a methodology to assess the risk associated with landslides in streams or river basins. The hazard posed by landslides is evaluated considering probabilistic methods that include the effects of rainfall and earthquakes. In addition, this study assesses the probability of a sliding mass reaching riverbeds and the probability of riverbed obstruction. Vulnerability is then estimated using impact curves based on the obstruction height. Finally, risk is estimated as the probability that economic losses occur along the riverbed. This methodology is based on probability methods, such as the first-order second-moment (FOSM) method, and the punctual estimates method (PEM). The methodology was applied in the La Liboriana River basin, in the municipality of Salgar in the northwestern Colombian Andes. On May 18, 2015, this mountainous and tropical area suffered a flash flood caused by landslides in the basin, which killed more than 100 inhabitants and caused infrastructure damage and significant economic losses. The results suggest that the proposed method coherently assesses the hazard posed by landslides and that the expected losses are comparable with the records from previous events.